This invention relates to a magnetic memory cell that has particular but not exclusive application to memory devices which include a memory array.
Hitherto, non-volatile random access memories (MVRAMs) have been proposed including magnetic random access memories (MRAMs) and ferroelectric RAMs (FRAMs), as candidates to replace conventional dynamic random access memories (DRAMs) and hard disc drives. A number of devices have been proposed which make use of giant magneto resistance (GMR) in multi-layers, as described in M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen van Dau, F. Petroff, P. Erienne, G. Gruzet, A. Friedrich, and J. Chazelas, Phys. Rev. Lett 61, 2472 (1988). Also, devices based on tunnelling magneto resistance (TMR) in magnetic tunnel junctions have been proposed as described in J. F. Bobo, F. B. Mancoff, K. Bessho, M. Sharma, K. Sin, D. Guarisco, S. X. Wang and B. M. Clemens, J. Appl. Phys. 83 6685 (1998). In these devices, information is stored in terms of the orientation of the magnetisation of small patterned structures and is read by measuring resistance. Previously proposed MRAM devices utilise in-plane magnetised magnetic layers together with a GMR or TMR measurement.
Another MRAM device has been proposed that utilises Hall effect measurements in a essentially two dimensional electron gas semiconductor multi-layer as described in F. G. Monzon, M. Johnson and M. L. Roukes, Appl. Phys. Lett. 71, 3087 (1997).
The use of in-plane magnetised magnetic layers imposes a restriction on the amount of downscaling that can be achieved in order to miniaturise the devices. Downscaling is limited by the increasing effect of the demagnetising field, generated by the magnetic sinks and sources of the magnetisation at the boundaries of the device. This causes a decrease in the effective magnetic anisotropy as the cell size is decreased, until the super-parametric limit is reached where the cell magnetisation becomes unstable at room temperature.
With a view to overcoming this problem, the present invention provides a magnetic memory cell comprising: an elongate, magnetic conductive element, a magnetic field producing device to apply a magnetic field to the conductive element, and a contact to allow detection of a Hall voltage developed across the element, the magnetic conductive element being configured to allow a magnetic domain to be induced therein and to provide pinning for its domain wall whereby the domain adopts a first configuration or a second different pinned configuration dependant upon the magnetic field produced by the magnetic field producing device, and such that first and second different values of the Hall voltage are produced for said respective domain wall configurations when an electrical current is passed through the magnetic conductive element.
The magnetic conductive element may comprise at least one planar layer of magnetic material with its easy or preferred axis of magnetisation extending transversely to the plane thereof, more particularly perpendicular to the plane. In this way, the influence of the demagnetising field is significantly less dependent on the lateral cell size than hitherto, which facilitates downscaling of the cell for purposes of miniaturisation. Layers of the magnetic material may be configured between layers of non-magnetic electrically conductive material. One material system may comprise layers of cobalt and platinum, which may be formed as a superlattice. Alternatively, xcfx84-Mn0.6-xNixAl0.4 where x less than 0.08 may be used.
The magnetic conductive element may include a region of reduced coercivity to promote formation of the induced magnetic domain. The region of reduced coercivity may be produced by focussed ion beam irradiation, for example using He+ ions at high fluency or Ga+ ions.